Electric wave transmission system



Feb. 1, 1944. J. G. KREER, JR.,ET AL ELECTRJ IC WAVE TRANSMISSION SYSTEM Filed Dec. 5 1942 2 Sheets-Sheet 1 1944- J. G. KREER, JR.,ET AL 2934@,$33

ELECTRIC WAVE TRANSMISSION SYSTEM Filed Dec. 5, 194:2 2 Sheets-Sheet 2 "f lm-J g Patented Feb. 1, 1944 T G F F l ELECTRIC WAVE TRANSMISSION SYSTEM John G. Kreer, Jr., Bloomfield, and Qharles O. Mallinckrodt, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 5, 1942, Serial No. 467,946

7 Claims. ('01. 178-44) 7 This invention relates to electric wave transmission systems and more particularly, although in its broader aspects not exclusively, to circuits for the automatic control of the characteristics of signal transmission systems.

An object of the invention is to increase the sensitivity, accuracy and stability of an automatic control system.

Another and more specific object of the inventlon is to provide improved and simplified circuit means for translating variations in the intensity of a pilot, or other current indicative of variations in the characteristics of a transmission system, into exaggerated or expanded variations in the intensity of a control current for effecting a compensating adjustment of the characteristics of the system.

The present invention will be disclosed herein as embodied in an organization for automatic regulation of repeater gain in a long distance wire line transmission system, such as-disclosed for specific example in an application of H. S. Black, Serial No. 467,947, filed of even dat herewith. In a system of this character, auxiliary or pilot waves are transmitted through the line concurrently with the signals, and the several pilot waves are utilized at infrequent repeater stations for controlling automatic adjustment of the transmission characteristics of the repeaters to compensate accurately for variations in the transmission equivalent of preceding sections of the system. If the system is a carrier telephone system operating in the frequency range from 12 to 60 kllocycles, for example, these repeaters may be located at intervals about one hundred miles for aerial cable and of about 250 miles for underground cables, and in the same system automatically regulated repeaters of simpler type may be located at intervals "of the order of fifteen miles. Whereas the-pilot wave power available at repeaters for controlling the gain adjustment thereof is comparatively feeble and its range of variation is necessarily minute, such variations in pilot intensity are to be accurately translated into magnified or expanded variations in the intensity of a control current that is strong enough to operate the gain adjusting mechanism.-

A feature of the present invention is an oscillator-amplifier circuit including a directly-heated oscillation-controlled thermistor that is heated principally by the pilot waves amplified therein and only to a small degree by the oscillations generated.

A further feature of the invention is a vacuum tube circuit that operates effectively as an pliefier is regulated by a thermistor ll under the' amplifier with respect to waves of pilot frequency and as an oscillator with respect to waves of a different frequency.

A still further feature of the invention is an oscillator-amplifier circuit arranged to compensate for variations in the mutual conductance of the oscillator tube.

A complete understanding of the novel arrangements contemplated by the present invention, as well as appreciation of the various features thereof, may be gained from consideration of the following detailed description and the annexed drawings in which:

Figs. 1 and- 2 illustrate schematically a repeater embodying the present invention: and

Figs. 3 and 4 show schematically simplified equivalent circuits of a part thereof.

Referring to Figs. 1 and 2 which should be as sembled with Fig. l above-Fig. 2, the repeater illustrated comprises in general outline a three stage amplifier of the stabilized negative feedback type having three variable impedance elements associated with the feedback path for automatically regulating the gain-frequency characteristic of the amplifier under the control of respective pilot waves transmitted over the line. In the input circuit of the repeater are fixed elements which eflectively equalize the transmission level over the frequency range of interest for an assumed normal line temperature. The amplifier is required to maintain the transmission level at its output substantially uniform over the frequency range and at a substantially constant predetermined value irrespective of line temperature changes and other effects tending to vary the transmission equivalent of the system. The gain-frequency characteristic it must have. to perform this function may be regarded as comprising three components of which the principal one, referred to as flat gain, is uniform over the frequency range. A second is a slope component which corresponds to a tilting of the gain-frequency characteristic: and the third is a curvature or bulge component.

The amount of flat gain introduced by the amcontrol of one of the three transmitted pilot waves. Any departure in the output intensity of that pilot wave from a preassigned value operates through a control circuit 11 to vary the temperature and resistance of thermistor H in such sense and amount as to effect a compensating change in the gain of the amplifier. Thermistor H, as well as other 'thermistors to be referred to subsequently, has a relatively large negative terntensity value at the output of the repeater. Thus 6 the'input electrodes of tube I33. The anode cirthe loss introduced under "the control of the three variable impedance elements is continuously denser Ill, and cathode resistor I6I.

adjusted so as to maintain the respective pilot levels substantially constant at the output of the amplifier. Such being the case, it will be evident that a very slight change in pilot intensity at the output of the amplifier must suffice to produce a relatively large percentage change in the resistance of its associated thermistor.

The provision of a thermistor in the feedback path of an amplifier for gain control purposes has been described in earlier patents, as for example Patent 2,179,915 issued November 14, 1939, to R. R. Blair. In the following detailed description it will be assumed that therepeater is incorporated in a l2-channel carrier telephone system utilizing line frequencies ranging from slightly'more than 12 kilocycles to slightly less than 60 kilocycles. It will be assumed further, that thejthree'pilot waves have frequencies of 12, 28 and 60 kilocycles per second respectively; and that they control the slope, curvature and fiat gain characteristics respectively.

Considering first theslope control circuit, it is to be noted that its function is to supply to the heater associated with thermistor MS a' heating current which varies widely in intensity in predetermined correlation with the variations in the intensity of the 12 kilocycle pilot wave appearing at the output of the repeater. r

The slope control circuit "comprises a single amplifying pentode tube I33 which is arranged to generate oscillations of comparatively low frequency for application to the heater associated with thermistor H5. The rate of change of intensity of the generated oscillations depends on the resistance of a current dependent thermistor I36 and will be zero,i. e., in a stable condition,

type filter and which is designed to pass only a narrow band of frequencies including '12 kilocycles. Resistance I32 acts as an input termination for filter I3l, while resistance I30 acts as an output termination of the filter and as a grid leak. The output terminals of filter l3I are connected to the control grid of pentode tube I33 and to the cathode thereof through an antiresonant circuit comprising an inductance (the secondary winding of transformer I ll) and con- The antiresonant circuit is tuned to the comparatively low oscillation frequency of 3.25 kilocycles, for

example, hence it offers negligible impedance to the efiective application of the pilot wave to cult of tube I33 comprises an antiresonant circuit made up of condenser I35 and reactors I34,

I31 and Ill, and a second antiresonant circuit comprising condenser H6 and the primary winding of transformer I43. The second of these antiresonant circuits is tuned to the oscillation frequency, hence it offers but little or negligible series impedance at the pilot frequency. The other antiresonant circuit is tunedto the pilot frequency, thereby presenting to tube I33 a high impedance of the orderof 100,000-ohms, and adapting the circuit for high voltage gain at the pilot frequency. The r-mistor I36 is connected across the series combinationof coil I31 and the primarywinding of transformer ll I. At the pilot frequency the latter presents negligible impedif thermistor I36 has a predetermined value such that'the gain around the oscillator loop is xactly -I and the phase shift is zero. The resistance of thermistor I36, in turn, is largely controlledin accordance with the intensity of the 12 kilocycle pilot which is taken from the output of the amplifier and applied to pentode I33 through filter l3l. The configuration of the control circuit and the proportioning of the various elements thereof are so arranged that the applied pilot waves are amplified with high efficiency and the power content of the amplified waves is dissipated substantially exclusively in the thermistor I36 to effect maximum control of the resistance thereof. In addition, the circuit configuration is such that the pentode I33 comprises an efilcient low frequency oscillator with. the intensity of oscillations stabilized and controlled by thermistor I36.

The 12 kilocycle pilot is picked off line 3|, which is connected to the anode of pentode 52, by band-pass filter l3l which-may be a crystal ancewhereas the impedance of coil I3! is made approximately equal to that of thermistor I36. Hence, substantiallyall'of the pilot power output of pentode I33-is delivered to anddissipated in thermistor I36. The circuit constants are so chosen that the pilotvoltage normallyappearing at the control grid of tube I33, will produce about 1.5 miiliwatts of power in thermistor I36.

The loss through transformer Ill at the slope pilot frequency is sufficiently great: that the amount of feedback at 12 kilocycles in the oscillator circuit is negligible. 7

Fig. 3 is a simplified equivalent circuit in which the elements that do not contribute to amplification of the pilot are omitted.

Considering now the elements of the control circuit that are significant at the oscillation frequency of 3.25 kilooycles, the simplified equivalent circuit takes the form illustrated schematically in Fig. 4. i

Transformer Ill is part of a feedback path provided to produce oscillations in pentode tube I33 at the selected oscillation frequency of 3.25 kilocycles which is, of course, substantially lower than the slope pilot of 12 kilocycles. At .this oscillation frequency capacitance I42 parallelresonates with the inductive component of transformer I ll resulting in the maximum possible feedback at the oscillation frequency.

At the oscillation frequency of 3.25 kilocycles the impedance of capacitance I35 is large compared to that of retard coil l3l so that thelatter has only a. minor effect on the loop. gain. Thermistor I36, on the other hand,'is connected directly across the oscillationfeedback path and its resistance has an important effect on the loop gain. The circuit constants are such that when the resistance of thermistor I36 is nominally about IOOO-ohms the loop gain is unity, i. e., will barely produce oscillations without building up. Accordingly, when the resistance of thermistor I36 is less than l000-ohms the circuit I assume 3 will cease oscillating and when the resistance is greater than 1000-ohms the oscillations will build up. V

The 3.25 kilocycle' oscillations produced by pentode tube I 33 are applied through output transformer I43 to line I44 and supplied there! over to the heater element of slope thermistor I45. Capacitance I46 parallel-resonates with the inductive component of transformer I43 to build up an impedance of the order of 50,000-ohms at the oscillation frequency of 3.25 kilocycles- This high impedance yields the maximum power from tube I33 in its linear operating range. v The impedance is small, however, at the 12 kilocycle pilot frequency so-that only a negligible amount of 12 kilocycle pilot power appears in the heated element of thermistor I45.

So long as the regulator circuit just described is oscillating at constant amplitude the resistance of thermistor I36 must-remain substantially constant at 1000-ohms and sincethe resistance is solely a function of the sum of the pilot power plus the oscillation power dissipated in it, it

follows that the sum of these two'powers must remain substantially constant while the circuit What actually happens is that when the pilot power changes, the oscillation is oscillating.

amplitudeautomatically adjusts itself until this condition is met. In one instance in practice,

the circuit described is so proportioned that the total power dissipated in the'thermistoris 1.63

milliwatts. In the same case the amount of pilot power represented in the total ranged from 1.5 to 1.6274 milliwatts during normal operation, corresponding tonormal variations in the pilot power at the output of the repeater of only plus and minus 0.18 decibel. The oscillation power represented in the total power dissipated in the thermistor therefore ranged from 0.13 to 0.0026 milliwatts; and the oscillation output power ranged from 50 to 1 milliwatts.

Thusa varia- III respect to the waves of pilot frequency, essentially an amplifier with thermistor I36 as load (as represented in Fig. 3) and, with respect towaves of oscillation frequency, it isessentially an oscillatorwith thermistor I36 loosely-coupled in the oscillator loop (as represented in Fig. 4)

Control thermistor 13s .is designed to be fast operating as compared to thermistor 146; as a specific example, the ratio may be of the order of 40:1. 'With such a ratio itis apparent that the heating and cooling time of thermistorlli will' have a negligible eilecton thecontrol circuit operation. The purposeof this is to reducetoa minimum the transmission variation "in the am plifier gain resulting'irom abrupt changes in thepilot level; the design of thecircuit is, therefore;

vsuch that theonly element having substantial eiiects on the restoring time of the regulator is thermistor I45. V The above discussion assumes that the 'trans conductance of oscillator tube I33 'will'hold constant at' its normal valu'e. In actual practice,

however, the transconductance will vary 'some-- what with battery changes and. aging. While these changes are reduced by a factor of about 2:1 by feedback through cathode resistor I 6|,

the effect of variations in transconductance is substantially minimized, inaccordance with a -feature..of the invention, by proper choice of the resistance versus power relationship for thermistor I36. As an illustration, let us assume that the. transconductance of tube I33-increases 1 decibel above its nominal value. This has two efl'ec'ts on the circuit operation, fir'st,the value of the resistance of thermistor I36 required to barely maintain oscillations is reduced and, second, the

' amount of pilot-power supplied to thermistor I36 tionin applied pilot power of about 8.5 per cent produced a 50 to 1 change in the strong control currents delivered to the slope thermistor I45.

Slope network I4! is designed'to provide proper regulating slope in the transmission efllciency of the amplifier, the amount of slope provided being determined by the resistance of slope thermistor I45; this resistance in turn dependsupon the strength of the oscillations supplied over line I44 which depends-in turn, as we havejust seen, upon the level of the 12 kilocycle slope pilot.

Varistors I5I and I52, together with biasing batteries I53 and I54, function as a limiter to prevent loss of control of the oscillations by thermistor I36 in the event'of certain abnormal disturbances in the pilotinput. a

Torecapitulate briefly'the operation of the follows that a small percentage of change in the latter will induce a' relatively large percentagechange in the former. a

In view of the planned design of the circuit of tube I 33 as outlined above, the circuit is, with is increased. The effect of increasing the'pilot power supplied to thermistor -I36is to' lower its resistance, and by designing thermistor I36 to have a resistance-power relationship such that' the increase in pilot power supplied duetoan increasein;transconductanceof tube I 33 reduces the'resistance-of thermistor|36 by exactly the amount necessary to maintain the gain around the loop at unity, thesetwo eifects can be made to cancel eachother and the output of control current for a given pilot input will be unchanged by the change intransconductance of tube I33.

The method of determining the required resistance-power relationship of thermistor I36 maybe described in detail v as follows: First, the valueof resistance at the position of thermistor I36 shall-be-such that the gain around theoscillator loop is exactly unity at the frequency for which the phase shift is. zero; and the proper resistance values are calculated for various values of transconductance of tube I33. Second, with these values of resistance assumed at that point- 601 and a fixed pilot input voltageassumed on the gridof tube I33, the pilot power dissipated in the assumed resistance is calculated for the same values'of transconductance'of tube I33. Third, with these same values of resistance and the-dc sired value of control power in-the output, the

pilot, power in the assumed resistance is cal-- culated. Fourth, these two values of power are added together and the total associated with the corresponding resistance gives the required char- 7 acteristic.

It may be noted that the calculations are carried out only for a specific value of pilot input which must be chosen as a matter of Judgment to correspond to the condition under which most accurate compensation is required. 7

, resulting from the operation of the fiat gain regu- Coming now to the operation of the bulge regulator, the operation and arrangement of the bulge regulator circuit is the same as that o! the slop regulator circuit described in detail above. The bulge regulator circuit has not been illustrated in detail, therefore, but is represented in the drawings by box I62. Thev bulge pilot frequency, which we will assume to be 28 kilocycles, is picked oi! line II by the bulge pilot filter oi circuit I02 and applied to the oscillator tube which is similar in arrangement and operation to tube I88 of the slope control circuit. The controlled oscillations are applied over line I68 to the heater of bulge thermistor I. Bulge network I85,

I similar in arrangement and operation to slope network I", is designed to provide proper regulating bulge in the transmission eificiency oi the amplifier, the amount of bulge provided being determined by the resistance oi bulge thermistor I; this resistance, in turn, depends upon the strength of the oscillations supplied over line I88 which depends, in turn, upon the level or the 28 kilocycle bulge pilot.

Considering now the fiat gain regulator, i. e.,

; the means for providing automatic and continuous regulation of the variations in the system transmission which are "fiat with respect to the nominal working range, thermistor II acts as the flat gain regulator, operating under control of the fiat pilot frequency control circuit now to be described. A predeterminedfiat pilot frequency, which we will assume to be 56 kilocycles, is picked-oil line 8i by band pass filter 82; filter 82 may be of the crystal type and is designed to pass only a narrow band of frequencies which includes 56 kilocycles.

If desired, an adjustable resistance pad may be included in line 8| to provide initial adjustment in order to compensate for minor manufacturing variations in the elements. Resistance 88 acts as a termination to: the input of filter 82.

The i6 kilocycle pilot selected by band pass filter if is transmitted by. tuned input transformer 88 to amplifier pentode tube 85. The pilot, "after amplification by pentode tube 85 in the usual manner, is transmitted by output transformer I to voltage-doubling rectifier bridge I8. The elements of the rectifier bridge act to convert the fluctuating pilot voltage impressed across the input terminals into a fiuctuating direct current voltage which appears across the output terminals. We will assume that the average normal output voltage of the rectifier is about 28 volts and is poled with the positive terminal toward the control grid of 5 kilocycle oscillator tube 86.

A bucking bias voltage of about 50 volts is introduced in series opposing relationship with the rectifier output voltage of +28 volts to provide a net grid bias of about 22 volts on oscil lator tube 88. Any increase in the output of rectifier bridge I8, resulting from increase in the intensity of the 56-kilocycle pilot, accordingly makes the grid bias of tube 86 less negative thereby increasing the mutual conductance of the tube and causing the oscillations produced thereby to increase in amplitude.

Rectifier 81 is provided to prevent the bias applied to the control grid of oscillator tube 88 from going positive; it limits the value of the direct current voltage obtained from rectifier bridge I8.

Capacitance SI and inductance 92 connected across the output terminals of rectifier bridge 18,

serve to suppress the transient changes in gain lator when it compensates for an abrupt change in input level. For example, if the elements 8| and 82 were not present in the circuit, the amplifier gain would tend to fluctuate after the fashion of a damped oscillation before stabilizing at its new value, when compensating for an abrupt change in input level. However, with elements SI and 92 in the circuit, the gain will change smoothly from its initial value to its final value.

Resistances IOI, I02 and I08 and varistor I04 which are included in the grid bias supply circuit of oscillator tube 88 aid in providing a favorabale input-output characteristic. More particularly, they tend to produce a linear relation between the applied control voltage and the oscillation output intensity, and to control the steepness of the input-output characteristic. Cold cathode tube I05 which is conductively associated with oscillator feedback transformer I06, is designed to break down at about 70 volts and thereby limit the 5 kilocycle power delivered to the heater winding of thermistor 1|.

The 5 kilocycle oscillations produced by oscillator tube 88 are applied through output transformer I01 and line III to the heater winding 'of thermistor I I, the fiat gain regulating element.

The circuit constants are such that a small change in the output from rectifier bridge I8 produces a large change in 5 kilocycle oscillation power produced by oscillator tube 88 and applied to the heater of thermistor 1|. The amount of 5 kilocycle power applied to the heater element, in turn, determines the resistance of the thermistor bead and inasmuch as the latter element is included in negative feedback path 51, it determines also the gain of the amplifier circuit. Thus the resistance of thermistor II, and therefore also the amplifier gain, changes in accordance with changes in the intensity of the 56 kilocycle pilot at the output of the amplifier, with the ultimate result that the flat gain of the amplifier is continuously and automatically controlled.

To recapitulate briefly the operation of the fiat gain regulator, tube 86 oscillates at a frequency of 5 kilocycles, the strength of the oscillations depending upon the difierential biasing voltage applied to its control grid; as this biasing voltage is obtained by bucking the 56 kilocycle pilot against a fixed direct current voltage, the strength of the oscillations depends upon the pilot level. Slight changes in the pilot level result in large changes in the strength of the 5 kilocycle oscillations, this being due to the expandor action of the oscillator tube and to the use of the difierential bias arrangement. may be 50 arranged that a change of 0.8 decibels in pilot level will result in thermistor 'II going over 16 decibels of its operating range.

Describing now the mu or amplifying circuit, input line section 2I includes series capacitance 22 and shunt inductance 23 which together provide a portion of the input impedance and are effective in providing a balanced input. Deviation equalizer 24 is intended to compensate for slight deviations of the gain-frequency characteristlc of a predetermined number of preceding amplifiers of the system from the desired characteristics.

A network represented schematically by impedances 25 and 28 is provided for adjusting lln e equalization and maintaining constant input resistance, this network being conductively associated with the primary side of input transformer 21. Input transformer 21 steps up the received As a specific example, the circuit carrier frequency voltages and applies them to the control grid of the first pentode vacuum tube 4| the input transformer in conjunction withassociated resistance 42, capacitances 43 and and inductance 45, has a sloping frequency characteristic sufficient to approximately equalize a predetermined length of the preceding line at average temperature. This portion of the input circuit may be designed for minimum losses in accordance with the disclosure of the copending application of H. W. Bode, Serial No. 435,171, filed March 18, 1942, entitled Coupling networks. Resistance 46 may be connected across the'line by key 41 in order to reduce the phase shift in the feedback circuit under certain conditions. 7

One side of the secondary winding of input transformer 21 is connected to the control grid of amplifying pentode tube 4| while the other side is connected through a feedback coupling impedance 48 to ground. .1 The cathode of pentode tube 41 and the cathode of the second pentode tube 5! are connected through respective capacitance-shunted resistors to ground while the cathode of the third pentode tube 52 is grounded through a capacitance-shunted resistor and battery 50 which supplements the plate battery. The suppressor grid of each of the three pentode tubes is conductively connected to its respectively associated cathode while the respective screening grids of the three tubes are capacitance bypassed to ground. The interstage coupling be tween pentode tubes 4| and 5| is of the impedance-capacitance type, the coupling elements being proportioned for a substantially flat gainfrequency characteristic over the frequency range of interest. The interstage coupling between pentode tubes 5| and 52 is predominately of the impedance-capacitance type with respect to the higher frequencies in the transmitted range and of the transformer coupled type with respect to the lower frequencies in the transmitted range, the elements again being proportioned forsubstantially flat'interstage gain.

Rectifier 53 is a substantially unilaterally conducting device conductively connected to the control grid of pentode tube 52 on the one hand and on the other hand over conductor 58 to the negative terminal of a biasing battery 54 and through the latter to the cathode of tube 52. Rectifier 53 is so poled as to allow flow of current through it only to the control grid of pentode 52, and the networks, and to the input side of the amplifier circuit. 1

While certain specific embodiments of the invention have been selected for illustration and detailed description, the invention isnot restricted in its application to such embodiments. The embodiments described should be looked upon as illustrative and not aszrestriictive.

- What is claimed is: Y 1. In a system for automatic regulation of the gain characteristics of a signal repeater under the control of pilot waves transmitted through the system, an electric wave amplifien-means for diverting pilot waves fromsaid repeater to said amplifier for amplification therein, a transmission loop circuit including saidramplifier for the generation of oscillations, a thermistor connected to receive and absorb .the pilot wave power output of said amplifier, circuit means whereby said thermistor is disposed also in oscillation stabilizing relation in said'transmission loop and supplied with oscillation power in an amount that is small compared with the pilot power supplied thereto, and means for utilizing the generated oscillations to adjust the gain characteristic of said repeater.

2. In combination, an oscillation generator, a source of control waves of variable intensity, a thermally variable impedance element connected in oscillation controlling relation to said generator, means for variably heating said impedance element under the control of said waves, and means for concurrently heating said impedance element with power derived from said oscillations, the derived heating power being small relative to both the total power content of said oscillations and the heating power supplied under the control of said waves.

3. A pilot controlled oscillator comprising an amplifier, means for applying pilot waves to be amplified to the input of said amplifier, a current dependent thermistor heated by the amplified pilot waves, anoscillation generating loop including said amplifier and said thermistor, and means for passing the generated oscillations through saidthermistor whereby the intensity of said osbiasing battery source 54 has such voltage that States Patent 2,210,001 issued August 6, 1940, to

E. H. Perkins entitled Electric wave amplifying system."

Negative feedback circuit 51 includes thermistor H which functions as described above to control or determine the amount of loss or attenuation in the feedback circuit. The feedback circuit 51 extends through the series arms of variable T-network 12, used for adjusting the amplifier gain, through impedance 14 provided for reducing interaction between the slope and bulge cillations is dependent on the intensity of the applied pilot waves, said amplifier being subject to changes in gain and resulting concomitant changes in the resistance of said thermistor and in the amount of heating power delivered by the amplified pilot waves to said thermistor, said thermistor having a power-resistance characteristic complementary to the relation between said concomitant changes in thermistor resistance and heating power whereby the effect of said changes in gain on the intensity of oscillations is substantially minimized.

4. A repeater gain control circuit comprising an amplifier, means for applying control current of variable intensity from the repeater to. the said amplifier for amplification therein, a loop circuitmcluding said amplifier for the generation of oscillations, a current dependent impedance element interposed in said loop circuit and connected to be traversed by both the generated oscillations and the amplified control current, and means for varying the gain of the repeater in response to variations in the intensity of the generated oscillations.

5. A control circuit adapted to translate small percentage variations in the intensity of a control current into large percentage variations in the intensity of a comparatively strong oscillation comprising means for amplifying the variable control current, an oscillation generator comprising said amplifying means, a current dependent impedance element connected to be traversed by the amplified control current and by a small proportion of the generated oscillations, the control current traversing said element being large compared with the oscillations traversing said element, and said impedance element being connected in oscillation stabilizing relation to said oscillation generator.

6. In combination, an amplifying discharge'device having input and output circuits, means for applying variable control current of a first frequency to the input of said amplifying discharge device, a positive feedback path for said amplithermistor, and an oscillation output circuit coupled to said second antiresonant circuit.

7. In a pilot controlled regulator, a vacuum tube circuit including an amplifier-oscillator vacuum tube, a thermo-variable impedance element connected in oscillation controlling relationship to said vacuum tube} a source of pilot frequency waves, the oscillation frequency of said vacuum tube being substantially different from the pilot frequency, and means for applying waves of said pilot frequency to-said vacuum tube, the frequency responsive characteristics of certain elements of said vacuum tube circuit being such that said circuit functions effectively with respect to waves of said pilot frequency as an amplifier with said impedance element as a load and functions effectively with respect to waves of said oscillation frequency as an oscillator with said impedance element loosely coupled in the oscillator loop.

Y JOHN G. KRE'ER, JR.

CHARLES O. MALLINCKRODT. 

